1. Field of the Invention
This invention relates to chromium-containing catalyst and more particularly to a chromium-aliphatic hydrocarbon catalyst containing an aluminum reducing agent, the preparation of such an olefin polymerization catalyst and the use thereof in the polymerization of olefins, especially C.sub.2 -C.sub.4 olefins.
2. Discussion of the Prior Art
It is known to polymerize ethylene in the presence of catalysts which comprise chromium (VI) oxides which are deposited on supports such as silica-alumina. For activation, these catalysts are heated, thereby effecting a reduction of the chromium (VI) oxide to lower chromium oxides. Reference is made to Houben-Weyl "Methoden der Organischen Chemie", Vol. 14/1, 1961, page 573. Activation of chromium (VI) oxides may also be effected by means of organic aluminum compounds such as aluminum trialkyls which contain 4 to 12 carbon atoms in the alkyl group. Reference is made to German Offenlegungschriften 2,209,737 and 2,314,412.
Besides chromium(VI) oxides, other chromium compounds such as chromium carboxylates, chromium acetates, chromic acid esters and chromium chelates have been used together with organoaluminum compounds as catalysts for the polymerization of ethylene. Thus, according to German Offenlegungschrift 1,906,589, ethylene may be polymerized in the presence of a catalyst which comprises chromium chloride which is adsorbed on an insoluble inorganic support and of an organoaluminum compound which contains not more than alkoxy groups attached to the aluminum metal atom. The polymerization is carried out at 30.degree. to 200.degree. C and 1.4 to 56 atmospheres in the presence of an inert organic solvent. Small amounts of additional alpha olefins or diolefins can be added to the ethylene to prepare the copolymers.
While catalysts of this type permit the polymerization of olefins, even on a commercial scale, their activity is not satisfactory in all cases. It has therefore become desirable to provide a chromium-containing catalyst system whereby the polymerization of olefins is characterized by high activity which in turn provides high yields of the desired polymer. It has also become desirable to provide a chromium catalyst system which will provide reasonably high molecular weight polyolefin polymers.